I think you are confusing the amount of acetaminophen (tylenol)
in vicodin with the amount of the narcotic, hydrocodone.
Vicodin 500
is the amount of the acetaminophen within the tablet. The doses of hydrocodone are 5mg, 7.5mg and 10mg. The 325, 500 or 650 is the amount of tylenol. Percocet and Percodan are the same narcotic, one just has acetaminophen and the other has aspirin.
There are other opiates between percocet and Oxycontin in strength.